Impact exploration is an important mode of deep space exploration. The high precision guidance law is the key to realize the impact mission. Based on the characteristics of deep space impact mission, the relative dynamic equations are derived and the pulse proportional guidance law is designed. In this paper, Monte Carlo simulation is carried out for the proportional guidance about the impact mission. Results show that the method comparing with the traditional predictive guidance method gets a slightly lower impact precision, but still can meet the demand of the mission. The velocity increment consumption of the method is basically the same with predictive guidance. On the other hand, the method can meet the requirements of the mission in different orbits, avoiding the limitation of autonomous navigation positioning accuracy to predict of guidance. This method can be applied in practice well.